1. Field of the Invention
The present invention relates to a light receiving apparatus for transmitting data between modules.
2. Description of the Related Art
As cell phones which have been generalized in recent years, folding-type, slide-type, or rotary-type cell phones have been frequently used for reasons associated with convenience. The folding-type, slide-type, and rotary-type cell phones are configured such that modules such as a communicating functional portion, amplifying circuit portion, and display circuit portion are separately arranged in a plurality of frames, and the wiring that connects these modules arranged in the frames is required to be flexible and strong enough to withstand driving.
The wiring between modules is further required to have a high-speed data transmission property, which is due to the multifunctionality of cell phones such as a music reproducing function, motion-picture reproducing function, and photographing function; prevent electromagnetic interference relative to external units; and consume low power, which is due to the desire for longer use between charging.
An attempt is being made to use optical fibers as wiring satisfying the above demands (for example, see Japanese Unexamined Patent Publication No. 2003-244295). A module performing optical data transmission by being connected to other modules with optical fibers will be hereinafter referred to as an optical communication module.
When using an optical fiber for wiring between modules, a light-emitting device for emitting light through an optical fiber and a light receiving device for receiving the light emitted through the optical fiber are mounted in a module, and data transmission is performed when the light receiving device receives the light emitted from the light emitting device and propagated through the optical fiber. For smooth data transmission, it is necessary to align the optical axis between the light emitting device and the optical fiber and the optical axis between the optical fiber and the light receiving device. In the following description, “to align optical axes” will be hereinafter briefly referred to as “alignment”.
For simple alignment, it is preferable to increase the core diameter of an optical fiber. With an increased core diameter of the optical fiber, however, when alignment is performed between a light emitting device with the optical fiber, due to an assembling error, light may come in from some other portion than the central axis of the optical fiber.
When the optical path of an optical fiber is long, even if light comes in from some other portion than the central axis of the optical fiber, the light is scrambled through the optical fiber during propagation, and thus a near-field pattern at the other end of the optical fiber becomes uniform. (Hereinafter, the “near field pattern” will be abbreviated to “NFP”.)
However, in the case of an optical fiber for connecting modules of a cell phone, where the optical path length is as short as several centimeters and thus the mode scramble effect is small, when light comes in from some other portion than the central axis of the optical fiber, the light is not sufficiently scrambled during propagation, and thus the NFP at the other end of the optical fiber becomes non-uniform. Even if light comes in from the central axis of the optical fiber, when there is a steep bend with a curvature radius of approximately 5 mm in the optical fiber, partiality occurs in the light intensity distribution of the light in accordance with the bend, and thus the NFP at the other end of the optical fiber becomes non-uniform.
There is a demand for a reduction in the diameter of the light receiving plane of a light receiving device in order to perform high-speed data communication. However, when the diameter of the light receiving plane of the light receiving device is small, the light receiving device cannot receive all of the light emitted from the other end of the optical fiber, but can only receive light from a portion of the other end plane of the optical fiber.
Therefore, when the core diameter of an optical fiber is large and the diameter of the light receiving plane of a light receiving device is small, the light receiving device may not receive a sufficient quantity of light because of a non-uniform NFP, depending on relative positions of the optical fiber and the light receiving device.
Moreover, because the state of the bend of an optical fiber fluctuates due to a folding or rotating operation of a cell phone, the NFP also fluctuates in accordance with the state of the bend.
Therefore, even if the accuracy of assembly of an optical fiber and a light receiving device is improved and alignment is performed according to the NFP, the light receiving device cannot receive a sufficient quantity of light and thus may not be able to stably transmit data.